Polyorganosiloxane compositions may be prepared by mixing constituents comprising polydiorganosiloxanes having curable or reactive groups with crosslinking agents and/or catalysts, as needed. Different types of curable groups may be used such as hydrolyzable groups, radical-curable groups, and/or heat curable groups. Generally, the polydiorganosiloxanes have greater than 2 reactive groups per chain. Compositions including these constituents can then be cured to make elastomeric materials by, for example, exposing the compositions to atmospheric moisture, radiation, or heat, depending on the curable groups present. Generally, the greater the number of curable groups on the polydiorganosiloxanes and/or the crosslinking agents, the greater the crosslink density of the resulting elastomeric material.
The cure rate of a particular composition depends on various factors including the type, specific structure, and number of reactive group(s) present. Within the same group type (e.g., an alkoxy or a carboxy group), different structures within a group have different reactivities (e.g., methoxy versus ethoxy within alkoxy group). For example, for hydrolyzable groups in the presence of moisture, a silicon-bonded acetoxy group will usually hydrolyze more rapidly than a silicon-bonded alkoxy group when all other conditions are the same. Furthermore, even groups of the same structure (e.g., two methoxy groups) can have different reactivities depending on the number of those curable groups bonded to a particular silicon atom. For example, if a polydiorganosiloxane has three silicon-bonded alkoxy groups bonded to one silicon atom on a chain end, then the first alkoxy group is generally most reactive (reacts most quickly). The structure of a polymer can also impact cure rate. For example, for a given number of reactive groups a higher concentration of reactive groups near the terminal of a polymer can lead to increased cure speeds as described in WO2014124364, and the accessibility of reactive groups can also impact cure speed. There is a continuing need to prepare polyorganosiloxanes with more efficient cure and faster cure rate by having more easily accessible reactive groups per molecule.
Polymer structure, chain length, and crosslink density all have an impact on physical properties of the final cured product. Furthermore, to show utility of the resulting cured product for specific applications such as certain silicone adhesive applications, reinforcing filler may be added to the composition to improve the physical property profile (e.g., increase tensile strength and increase % elongation to break). As an alternative example, for applications where heat management is required, thermally conductive filler may be added to the composition to provide thermal conductivity to the composition and so forth.